Method for measuring surfaces by confocal microcopy

ABSTRACT

The invention relates to a method for measuring surfaces by confocal microscopy using the reflection method, specially to measure the superficial profiles ( 1 ) of treated or drilled teeth ( 2 ). The invention seeks to eliminate mistakes occurring when very inclined areas are measured. To this end, the method disclosed is characterized by a confocal representation with enhanced dynamics (relative sensitivity) enabling it to project both the retro-reflections and the weak scattered light ( 3 ) or fluorescent light of each focal plane ( 8 ).

BACKGROUND OF THE INVENTION

The invention concerns a process for surface measurement using confocalmicroscopy in a reflection process, especially for measuring the surfaceprofile of teeth in the untreated and in the treated or drilledconditions.

This is essentially a process for measurement of surfaces of any typeand any contour. Various processes for surface measurement are alreadyknown in practice.

For instance, a line of light can be projected on the object with alight sectioning sensor, and observed at an angle with a CCD camera. Thegeometric deformation of the line is measured. The height differences onthe object are computed from this deformation. By moving the objectunder the sensor—perpendicularly to the light line—and by repeatedmeasurement, a surface form can be measured or determined from a seriesof profiles.

The light-sectioning sensor is indeed a simply designed and thus arobust sensor, but the oblique lighting which it requires causesunilateral shading of steep surfaces. That causes asymmetries in theimaging, or inaccuracies. Furthermore, error can be introduced into themeasurements because of scattering of light from various depths of, forinstance, an at least partially transparent tooth material.

Furthermore, it is already known in practice that surfaces can bescanned with confocal microscopy so as to generate three-dimensionalpictures of the surface. In this respect, it is only necessary to referto an article by J. Engelhardt and W. Knebel in Physik in unserer Zeit′[Physics in Our Time], Vol. 24, 1993, No. 3, titled “ConfokaleLaserscanning-Mikroskopie” [Confocal Laser Scanning Microscopy], and oneby D. K Hamilton and T. Wilson in Appl. Phys., B27, 211-213, 1982,titled “Three-dimensional Surface Measurement Using the ConfocalScanning Microscope”. Confocal microscopy is very specially suited tosurface measurements of tooth surfaces, because this process images onlythose structures which are exactly in the focal plane of the microscopeobjective. Thus measurement errors due to the partially transparenttooth material are effectively avoided. To be sure, the method ofreflection measurement with the usual confocal microscope fails at steeptransitions or flanks if their angle is greater than the aperture angleof the objective, because then the reflection no longer enters theobjective, and is lost for evaluation. (See P. C. Cheng and R. G.Summers in: “Handbook of Biological Confocal Microscopy”, Plenum Press,New York, 1989, Chapter 17.)

SUMMARY OF THE INVENTION

Therefore this invention is based on the objective of providing aprocess for surface measurement with which it is possible to measuresurfaces of partially trasparent materials, and surface profiles withsteep flanks, without problems. This process should be very particularlysuitable for use in dentistry, that is, to measure the surface profileof teeth in the untreated and in the treated or drilled conditions.

The surface measurement process according to the invention attains theobjective stated above by a process for surface measurement usingconfocal microscopy in a reflection mode, especially to measure thesurface profile of treated or drilled teeth, which is characterized byconfocal imaging with high dynamic range (relative sensitivity) so as toimage reflections and also weak scattered light or fluorescent lightfrom the particular focal plane.

It is learned here for the first time, according to the invention, thatconfocal microscopy is quite particularly suited to surface measurementof partially transparent materials, because in confocal microscopy onlythose structures which are exactly in the particular focal plane of themicroscope objective are imaged. It is also learned that thedisadvantage of ordinary reflection confocal microscopy with respect tothe aperture problem mentioned above can be eliminated by utilizingscattered light or fluorescent light from the particular focal plane forthe usual evaluation of the reflection.

The evaluation of the scattered or fluorescent light can be accomplishedin a further manner according to the invention through confocal imagingwith a high dynamic range, i.e., with high relative sensitivity, so thatit is possible to image both strongly reflecting flat surfaces and alsoto show the scattered or fluorescent light even on steep flanks.Accordingly, imaging is possible by the process according to theinvention even if the light reflected from steep flanks misses theobjective so that, in the usual reflection process, no profilometry canbe done. Finally, the scattered light is always used for evaluation ifimaging is no longer possible in the absence of specular reflections bythe usual confocal microscopy.

As already mentioned above, the detector signal is digitized at highresolution, particularly advantageously with a dynamic rangesubstantially greater than 8 bits. The relative sensitivity, or dynamicrange, of the confocal imaging can be 16 bits for very particularlyeffective utilization of the weak scattered light or fluorescent lightin the vicinity of steep surface slopes.

An algorithm is provided to evaluate elevations, or to produce thesurface profile, using weak scattered light. It takes intoconsideration, or tolerates, the high dynamic range of the system. Thisalgorithm takes the nearest, or indirectly adjacent focal planes intoconsideration by interpolating, with the higher intensities in the localregion being relatively over-weighted so as to reduce the dependence onthe background signals. Finally, a suitable algorithm is provided, sothat, after detection of the scattered light signal and afterhigh-resolution digitizing, an adequate height evaluation can be madefrom the digitized signal.

It must be emphasized here that the surfaces can also be scanned with adark-field system. Either a point light source or a light sourceappropriately diaphramged can be provided.

In the area of application to dentistry, and particularly to producingexactly fitted inlays instead of the usual amalgam fillings, it is veryspecially advantageous first to scan the surface of the untreated toothand to store the detected values, preferably digitized and alreadyprocessed to give the height profile. In the next step the tooth istreated or drilled. Then the treated or drilled tooth is scanned again,again with storage of the values giving the surface profile of thetreated tooth. From the difference between the two surface profiles, orfrom the values across the surface profile, the surface, or the exactmeasurements, are calculated for the inlay required so as to giveoptimal occlusion of the treated tooth.

It is highly advantageous, to get particularly high precision inprocessing the inlay, if the inlay being produced is scanned after aninitial processing, and if the further processing is done by means ofcorrection values obtained by a comparison of the actual and desiredvalues. Correction to verify the inlay shape is possible to the extentthat, with repetition of this process, high precision is possible inproducing the inlay and optimal occlusion is possible. The measuresdescribed above also allow consideration of inaccuracies caused by theequipment or the tools, such as tool wear, to be taken intoconsideration so that optimal fitting of the inlay and thus optimalocclusion are possible even with a tolerance range at the processingstation.

It is also possible that, in a subsequent step, the cavity produced inthe tooth may be filled with a plastic composition so that, when thepatient bites on it, the contact points with the opposing teeth aremarked in the plastic mass. Then the surface profile generated in thatway is scanned, the measurements obtained with respect to the surfaceprofile are stored, and they are taken into consideration in calculatingthe surface or dimensions of the inlay to be produced.

BRIEF DESCRIPTION OF THE DRAWINGS

Now there are various possibilities for designing and developing theteaching of this invention in an advantageous manner. Reference is madeto the following explanation of one example embodiment of the inventionby means of the drawing. The generally preferred designs anddevelopments from the teaching are explained in connection with theexplanation of the preferred example embodiment of the invention. Thedrawing shows:

FIG. 1 an application of the process according to the invention in aschematic representation of one example embodiment, and

FIG. 2 a schematic representation of another example embodiment of theprocess according to the invention with a dark-field system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The figures show schematically the principle of an example embodiment ofthe process according to the invention for surface measurement usingconfocal microscopy, with the specific case of measurement of thesurface profile 1 of a tooth 2 indicated schematically.

The invention provides confocal imaging with a high dynamic range, i.e.,with high relative sensitivity, for imaging both reflections and weakscattered light 3 from the particular focal plane. The detected signalis digitized with high resolution with a dynamic range greater than 8bits.

According to the depiction in FIG. 1, light emitted from a light source4, or the light beam 5, impinges on a beam splitter 6, and from there itis directed through an objective lens 7 onto the surface or surfaceprofile 1 of the tooth 2. In the depiction selected here, the focalplane 8 lies on a flank which is so steep that the specularly reflectedbeam 9 no longer strikes the objective lens 7. That is because the angleof the flank in this case is greater than the aperture angle of theobjective lens 7.

FIG. 1 also shows that the non-specular diffuse scattered light 3 fromthe focal point 10 passes to or through the objective lens 7 and throughthe beam splitter to the detector 11. For simplicity, any parts such aspinholes or diaphragms or the like in the light path are omitted fromthe depiction. In any case, measurements from the detector 11 aredigitized at high resolution and then submitted to a height evaluationto obtain the surface profile 1. The digitizing and height evaluationcan be done by an electronic control system 12 or by a computer 13.

The example embodiment shown in FIG. 2 differs from that of FIG. 1 byshowing an application of confocal microscopy in a dark-field system.The light beam 16 from a point source of light 15 goes from a reflector17 through the objective lens 7 to the surface of the tooth 2. The lightbeam strikes a surface which is slanted so that the specularly reflectedbeam 18 does not return to the objective lens 7. Otherwise, to avoidrepetitions, see the description for FIG. 1.

Finally, it should be noted that utilization of weak scattered light orfluorescent light is necessary for confocal imaging by the processaccording to the invention. That makes possible high sensitivity, or ahigh dynamic range, for evaluating the scattered light and forprofilometry, at least including the scattered light at steep flanks.

List of Reference Symbols

1. Surface profile

2. Tooth

3. Scattered light

4. Light source (FIG. 1)

5. Light beam (FIG. 1)

6. Beam splitter

7. Objective lens

8. Focal plane

9. Specularly reflected light (FIG. 1)

10. Focal point

11. Detector

12. Electronic control system

13. Computer

15. Point source of light (FIG. 2)

16. Light beam (FIG. 2)

17. Reflector

18. Reflected beam (FIG. 2)

What is claimed is:
 1. A method of measuring a surface of a tooth usingconfocal microscopy in a reflection process for purposes of a dentalinlay, said method comprising the steps of: (A) scanning said surfaceprior to treatment of said tooth to detect a series of images of bothreflections and weak scattered light or fluorescent light from saidsurface in a particular focal plane by confocal imaging with highdynamic range to generate a signal; (B) generating a three-dimnensionaldigitized representation of said surface of said untreated tooth fromsaid signal using an algorithm which takes said high dynamic range intoconsideration, and storing said three-dimnensional digitizedrepresentation of said surface of said untreated tooth; (C) repeatingsteps (A) and (B) with respect to a surface of said tooth after a holehas been drilled in said tooth to store a three-dimensional digitizedrepresentation of said surface of said drilled tooth; and (D)calculating a three-dimensional digitized representation of a desiredsurface of said inlay from the difference in said three-dimensionaldigitized representations of said surface of said treated tooth and saidsurface of said drilled tooth.
 2. The method according to claim 1,further comprising the steps of filling said drilled hole with adeformable compound so that contact points with opposing teeth aremarked by biting on said compound, repeating steps (A) and (B) withrespect to a surface of said tooth and marked compound to store athree-dimensional digitized representation of said surface of said toothand marked compound, and using said three dimensional digitizedrepresentation of said surface of said tooth and marked compound incalculating said three-dimensional digitized representation of saiddesired surface of said inlay.
 3. The method according to claim 2,further comprising the steps of forming said inlay based on saidthree-dimensional digitized representation of said desired surface ofsaid inlay, repeating steps (A) and (B) with respect to a surface ofsaid actually formed inlay to store a three-dimensional digitizedrepresentation of said surface of said actually formed inlay, comparingsaid three-dimensional digitized representation of said surface of saidactually formed inlay with said three-dimensional digitizedrepresentation of said desired surface of said inlay, and furtherforming said inlay to reduce differences between surface of saidactually formed inlay and said desired surface of said inlay.
 4. Themethod according to claim 1, further comprising the steps of formingsaid inlay based on said three-dimensional digitized representation ofsaid desired surface of said inlay, repeating steps (A) and (B) withrespect to a surface of said actually formed inlay to store athree-dimensional digitized representation of said surface of saidactually formed inlay, comparing said three-dimensional digitizedrepresentation of said surface of said actually formed inlay with saidthree-dimensional digitized representation of said desired surface ofsaid inlay, and further forming said inlay to reduce differences betweensurface of said actually formed inlay and said desired surface of saidinlay.